The zippo package

This light-weight library provides a basic, but fully-type-checked
zipper implementation, suitable for any algebraic data structure.
Our implementation has no notion of "left" and "right", and
instead uses lenses (from the "yall" package) to indicate directions
to "navigate down to".

For a similar approach with many more features, but less type-safety
see "pez".

UPDATE: I have stopped developing this package; instead look into "lens" v3.0 which includes similar functionality